In 1874 F. Braun observed that “bei einer grosen Anzahl naturlicher und kunstlicher Schwefelmetalle … der Widerstand derselben verschieden war mit Richtung, Intensitat und Dauer des Stromes”. W. Schottky then, in 1938, explained the rectifying behaviour of such metal-semiconductor contacts by a depletion layer which is characterized by the “Austrittsarbeit der Uberschus- oder der Defektelektronen an der Grenze Metall-Halbleiter”. Mott and most probably also Schottky postulated that the barrier height, as it is called now, equals the difference of the work function of the metal and the electron affinity of the semiconductor. Experimentally, the barrier heights are not found to obey this simple rule. The deviations were first assigned to electronic interface states by Bardeen. Following a later proposal of Heine's, the metal wave-functions tail into the virtual gap states of the semiconductor and by this determine the barrier height at the metal-semiconductor interface. This model very well describes all chemical trends observed with the barrier heights of metal-semiconductor contacts.